1. Field of the Invention
The present invention relates to a process for the preparation of dialkyl carbonates by reacting carbon monoxide with alkyl nitrites in the presence of a heterogeneous catalyst using a fixed bed reactor with a short catalyst bed in the direction of flow.
Dialkyl carbonates are of general chemical and industrial importance. Thus, for example, diethyl carbonate is an excellent solvent in the medium boiling range. Dialkyl carbonates are also excellent carbonylating and acylating reagents. They are of great importance in the preparation of other carbonates, urethanes and ureas. Finally, on account of their high oxygen content, they are suitable as fuel additives for improving the knock rating of motor fuels.
2. Description of the Related Art
It is known to prepare dialkyl carbonates by reacting phosgene or alkyl chloroformates with alcohols.
There is an increasing interest, however, in superseding the use of the toxic phosgene or the intermediates derived therefrom, such as chloroformic acid esters, by other processes.
Particularly important processes here are those in which carbon monoxide is reacted in the gas phase with alkyl nitrite on a platinum metal catalyst. This process is based on equation (1) (using dimethyl carbonate as an example). ##STR1##
Methyl nitrite itself can be produced for this purpose in a manner known per se, in an upstream reaction according to one of equations (2)-(5). EQU 4NO+O.sub.2 +4CH.sub.3 OH.fwdarw.4CH.sub.3 ONO+2H.sub.2 O (2) EQU NO+NO.sub.2 +2CH.sub.3 OH.fwdarw.2CH.sub.3 ONO+H.sub.2 O (3) EQU N.sub.2 O.sub.4 +CH.sub.3 OH.fwdarw.CH.sub.3 ONO+HNO.sub.3 ( 4) EQU 2NaNO.sub.2 +H.sub.2 SO.sub.4 +2CH.sub.3 OH.fwdarw.2CH.sub.3 ONO+Na.sub.2 SO.sub.4 +2H.sub.2 O (5)
The preparation of dimethyl carbonate by reacting carbon monoxide and methyl nitrite in the gas phase in the presence of a heterogeneous catalyst, preferably a supported catalyst containing a platinum metal, particularly preferably a supported catalyst containing palladium and very particularly preferably a supported catalyst containing a palladium halide, has been variously described, for example in the following scientific publications or patent publications:
JP 60/181 051; X.-Z. Jiang et al., Cuihua Xuebao 10(1), 75-78 (March 1989);
EP 425 197; X.-Z. Jiang, Platinum Metals Rev. 34(4), 178-180 (1990); EP 464 460; EP 503 091; EP 501 507; EP 503 618; EP 523 508; EP 523 728; EP 538 676.
One technical embodiment of this process is described in patent application EP 523 728. This Patent Application comprises the recycling of the nitrogen oxides released in the course of the reaction of methyl nitrite with carbon monoxide, together with the unreacted gaseous reactants and the additional gas required for rendering the system inert, preferably nitrogen, into a process step upstream of the actual dimethyl carbonate production process, which process step corresponds to equation (2) and in which the methyl nitrite required for the reaction is regenerated by feeding in methanol and oxygen and removing to the greatest possible extent the water released in this step. This is therefore a cyclic process in respect of the gaseous components involved, namely in respect of the inert gases and auxiliary substances, the unreacted gaseous reactants, for example the unreacted methyl nitrite and carbon monoxide, and the nitrogen oxides involved.
The actual process for the preparation of dimethyl carbonate for example, described by equation (1), takes place on a heterogeneous catalyst located inside a tube bundle reactor. A disadvantage here is the high thermal stress on the catalyst resulting from the substantial heat of the reaction between carbon monoxide and methyl nitrite which proceeds according to equation (1).
The thermally labile methyl nitrite is readily decomposed under these conditions.
It is therefore hardly surprising that 0.5 wt % of formaldehyde dimethylacetal, based on the dimethyl carbonate formed, is recovered in the crude product in the case of the procedure described in said Patent Application EP 523 728. For some intended applications of dimethyl carbonate, however, such impurities are unacceptable and extensive separation and purification steps are therefore necessary.
If such substances are low-boiling, as for example in the case of said formaldehyde dimethylacetal, further problems arise due to the inevitable accumulation of such volatile components within the whole of the basic cyclic process. To prevent the concentration of by-products within a process operated as an industrial cyclic process, specified proportions of the circulating gas and condensed reaction products, except for dimethyl carbonate itself, must be withdrawn continuously or batchwise, preferably continuously. This causes raw material losses and requires expensive off-gas treatments.
The object was therefore to carry out the reaction of the gaseous reactants carbon monoxide and alkyl nitrite on a heterogeneous catalyst in such a way that the heat of reaction thereby released is rapidly dissipated, thereby minimizing the formation of methyl nitrite decomposition products.
Continuous thermostatic control should be arranged for reactions with an extremely high heat tonality and catalysts or reactions with an extremely sensitive temperature behaviour.
Here the catalyst can be accommodated for example either between the tubes or else inside the tubes of a heat exchanger (a Linde reactor according to German Offenlegungsschrift 34 14 717 or a tube bundle reactor according to Chemie-Ingenieur-Technik 51 (1979), pp. 257-265).
Reactors of this type with tube diameters of one to several cm and tube lengths of 2-20 m have been state of the art for a long time.
Despite the constant flow of heat in the radial direction, a hot spot may be formed inside the reactor tubes, especially in larger reactors, for example in the synthesis of dimethyl carbonate from methyl nitrite, this being responsible for losses of selectivity as a result of the undesired formation of formaldehyde dimethylacetal.